Mars is Melting

Mars is Melting

August 7, 2003: It's
not every day you get to watch a planetary ice cap vanish, but
this month you can. All you need are clear skies, a backyard
telescope, and a sky map leading to Mars.

Actually, you won't need the sky map because Mars is so bright
and easy to find.

Just look south between midnight and dawn on any clear night
this month. Mars is that eye-catching red star, outshining everything
around it. It's getting brighter every night as Earth and Mars
converge for a close encounter on August 27th.

Above: Amateur astronomer Thomas
Williamson of New Mexico took this picture of Mars on August
1st. He used an 8-inch telescope and a digital web camera. [more]

Mars has gotten so big in recent weeks that even a backyard
telescope will show details on the planet's surface: dust clouds,
volcanic terrains, impact basins. Best of all is the south polar
cap. Made of frozen CO2 or "dry ice," it
reflects more sunlight than any other part of the planet. The
southern hemisphere of Mars is tipped toward Earth and the bright
cap is remarkably easy to see.

Don't wait too long to look, though, because the ice will
soon be gone.

Like Earth, Mars has seasons that cause its polar caps to
wax and wane. "It's late spring at the south pole of Mars,"
says planetary scientist Dave Smith of the Goddard Space Flight
Center. "The polar cap is receding because the springtime
sun is shining on it."

As the cap shrinks it develops rifts, dark spots, and a ragged
border. Lately, for instance, amateur astronomers using 8-inch
and larger telescopes have been watching a frosty mountain range
emerge from the ice. Says Smith, "these are the Mountains
of Mitchel"--named after the Ohio astronomer who first spotted
them 150 years ago. A bold dark rift called Rimas Australis
cuts through the polar ice just south of those mountains. (These features
are visible in Thomas Williamson's photograph of Mars at the
beginning of this story.)

Something else to look for is the "Cryptic
region"--a dark zone hundreds of km wide. Even after the
ice above it recedes, the Cryptic region remains remarkably cold
according to infra-red cameras onboard NASA's Mars Global Surveyor
spacecraft. No one is sure what the Cryptic region is, "but
it's probably big enough to see from Earth," notes Smith.

Left: A brightness map of the martian south pole one
Mars-year ago. The Cryptic region is the blue-green area around
the 4 o'clock position. Reds and yellows denote frozen CO2. This
map was created by Dave Smith using data from the 1 micron detector
of the laser
altimeter onboard Mars Global Surveyor. [enlarge]

Here's an amazing fact: The seasonal polar caps are made of
martian air that freezes during winter. Depending on the time
of year, more than a quarter of the martian atmosphere can be
found lying on the ground around the poles. (The atmosphere is
95% CO2; that's why the seasonal polar caps are made
of dry ice.)

As seasons come and go, carbon dioxide shifts back and forth--lying
on the ground during cold months, floating through the air during
warmer months. The world-wide air pressure rises and falls by
25%.

For comparison, the air pressure inside a hurricane on Earth
is often only a few percent lower than ambient. You can experience
a full 25% difference in pressure by traveling from sea level
to the top of a 9000 ft (3000 m) mountain. Just try running a
100 yard dash up there.

Right:
The ups and downs of air pressure on Mars recorded by NASA's
Viking Landers. [more]

The south polar cap is vaporizing now, which means CO2
is rushing back into the atmosphere. "Remember, though,"
adds Smith, "there are two polar caps on Mars--north and
south. While the south polar cap is vaporizing the north polar
cap is growing. It's a balancing act. Overall air pressure will
be greatest when there's the least amount of CO2 on
the ground." The next such peak is due in early October--that
is, early southern summer on Mars.

The boost in pressure has some interesting consequences. It
won't make the martian atmosphere thick by Earth-standards. At
best the air pressure on Mars is 100 times less than Earth. But
it might become thick enough in some places for liquid water
to flow.

NASA spacecraft have detected frozen water beneath the surface
of Mars. Liquid water, on the other hand, is scarce. Why? On
a warm summer day, ice doesn't melt, it vaporizes--skipping directly
from solid to gas. This happens because the air pressure is so
low. But a small boost in pressure could be enough to allow ice
to melt and water to flow under a warm summer sun. Southern summer,
therefore, might be a good time for future human explorers to
visit. (For more information read Science@NASA's Making
a Splash on Mars.)

On the other hand, thicker air also encourages dust storms,
which are a big problem on Mars. Small dust clouds stirred by
sun-warmed winds sometimes grow to encircle the entire planet.
In 2001 such a storm lasted for months and frustrated astronomers
who couldn't see through the haze.

Will that happen again this year? No one knows.

Below: In early August, look for bright Mars rising
above the southeastern horizon after 10 p.m.. The planet is even
easier to find between midnight and dawn, when it hangs high
and bright in the southern sky. (These instructions apply to
observers at mid-northern latitudes.)

When the seasonal polar cap finally vanishes, Smith recommends
looking for the permanent polar cap. "The permanent
cap is made of frozen water hiding beneath the seasonal cap of
CO2," he explains. While the seasonal cap is
wide-ranging (90o to 60o latitude) and
shallow (only 1-meter deep), the permanent cap is compact and
about 3-km deep. "It harbors a mass of water comparable
to the mass of the martian moon Phobos." To amateur astronomers
peering through telescopes, the water-ice cap will look like
a tight white knot within 10o latitude of the pole.

Dark "cryptic" spots. Mountainous rifts. A treasure
trove of water. There's a lot to look for around the south pole
of Mars. Grab a telescope and see for yourself!

Editor's note: In this story we talk about the martian
polar caps "melting" or "vaporizing." That's
not exactly right. A physicist would say more accurately "the
polar caps are subliming." In other words, the frozen CO2--better
known as "dry
ice"--transforms directly from a solid to a gas without
going through an intermediate liquid phase.

Right: The linear ridge at the 1 o'clock position
traces the Mountains of Mitchel. This brightness map of the martian
south pole was obtained months after the one that appears earlier
in this story--hence the different appearance of the polar cap.
Credit: MOLA/Mars Global Surveyor. [enlarge]

Making a Splash on Mars -- (Science@NASA) On a planet that's
colder than Antarctica and where water boils at ten degrees above
freezing, how could liquid water ever exist? Scientists say a
dash of salt might help.

Once Upon a Water Planet -- (Science@NASA) Today the Red Planet
is dry and barren, but what about tomorrow? New data suggest
that the long story of water on Mars isn't over yet.